1. Field of the invention
The present invention relates to a selective catalytic reduction for the removal of nitrogen oxides (NO.sub.x) and a catalyst body thereof. More particularly, the present invention relates to a selective catalytic reduction for the removal of nitrogen oxides, prepared from a spent catalyst composition from heavy oil upgrading facilities of an oil refinery after a hydro-desulfurization process, superior in thermal and chemical stability and economically favorable in production cost and a catalyst body comprising the same.
2. Description of the Prior Art
Nitrogen oxides are inevitably generated in the plants which employ fossil fuels, such as power plants and chemical plants. Nitrogen oxides are found to be an immediate cause of the air pollution, such as acid rain and smog, and the permissible discharge standard, therefore has recently been strengthened.
Generally, nitrogen oxides are generated upon the oxidation of nitrogen monooxides in a high temperature combustion equipment or the reaction of nitrogen with oxygen in an excess of air. To eliminate the root of nitrogen oxides, there have been made many attempts for the improvement of the combustion condition, such as low oxygen combustion and exhaust gas circulation. However, the nitrogen oxides cannot be completely eliminated only by the improvement in combustion technique and thus, there are developed and suggested various post-treatment techniques by which the exhaust; gas is deprived of nitrogen oxides.
Depending on whether a solution is used or not, techniques for denitrifying exhaust gas are classified generally into two: wet method and dry method. Now, it is known that the latter is more advantageous in investment, operation cost, and the treatment of secondary waste than the former. Of them, the most frequently used technique is the selective catalytic reduction in which nitrogen oxides are decomposed into water and nitrogen in the presence of a catalyst using ammonia as a reducing agent.
As many as tens of selectively reducing catalysts have been disclosed in patents, thus far and they are very various in types and shapes. According to a general classification, there are catalysts containing the precious metals of the platinum group, catalysts containing the oxides of metals, such as iron, cobalt, nickel and molybdenum, zeolite catalysts containing ion-exchanged copper, and catalysts consisting mainly of vanadium and titanium.
U.S. Pat. No. 3,993,572 discloses a catalyst containing a metal of the platinum group, a rare-earth metal and alumina, which is impregnated in a carrier such as ceramic honeycomb.
U.S. Pat. No. 4,018,710 discloses a catalyst comprising a cerium oxide and a nickel oxide in combination with various amounts of rhodium or ruthenium. The patent also suggests that a metal sulfide, such as copper sulfide or potassium sulfide, may be added to the catalyst in order to prevent ammonia from occurring.
U.S. Pat. No. 4,052,337 discloses a catalyst comprising a natural or synthetic zeolite ion-exchanged with copper or alkali metal, which is useful to remove nitrogen oxides in the presence of ammonia.
In U.S. Pat. No 4,010,238 is disclosed that the nitrogen oxides are selectively removed from exhaust gas by use of a metal oxide catalyst in which vanadium is combined with one selected from the group consisting of copper, zinc, tin, lead, titanium, phosphorus, chrome, iron, cobalt and nickel.
U.S. Pat. No. 4,048,112 introduces a catalyst comprising a vanadium oxide impregnated in a titanium oxide carrier of an anatase form, with which nitrogen oxides can be removed out of exhaust gas in the presence of ammonia.
U.S. Pat. No. 4,085,193 describes a catalyst in which titania is combined with at least one selected from the oxides of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chrome and uranium.
Although the catalysts and processes described in the above-cited patents are well known in the aspect of technical perfection and commercialization and frequently put in practice, it is widely recognized the need of a more improved catalyst when considering the thermal resistance required for the treatment of hot exhaust gas, the durability to the heavy metals or sulfur oxides of exhaust gas, the secondary pollution attributable to the use of catalyst and ammonia, and economical profit.